A Diesel-cycle engine can be fueled with natural gas instead of diesel to obtain emission and economic benefits. In these engines a diesel pilot fuel is often employed since the auto-ignition temperature of natural gas is greater than that of diesel. A small amount of diesel, normally around 5% of total fuel introduced to the combustion chamber, is injected along with the main natural gas fuel. The diesel ignites due to compression heat and subsequently the natural gas is ignited due to the combustion of diesel.
Air-fuel ratio is an important engine parameter that influences emissions and fuel usage of gaseous fueled engines. Air-fuel ratio can be expressed as the excess air ratio which is the ratio of the actual air-fuel ratio to the stoichiometric air-fuel ratio (when all the oxygen and fuel are consumed during combustion). The excess air ratio can also be referred to as lambda (λ). Another parameter related to air-fuel ratio if the fuel-air ratio, which is simply the inverse. Similarly, the equivalence ratio is the ratio of the actual fuel-air ratio to the stoichiometric fuel-air ratio. As used herein these terms are used interchangeably to describe the relationship of air with fuel in the combustion process. In premixed engines the excess air ratio is controlled to within a narrow range of the stoichiometric value (1). Conventional diesel-cycle engines that fuel from diesel typically operate lean and with a broad range of excess air ratios, for example with lambda somewhere between 1.3 at full power and 10 at idle. Gaseous fueled engines can operate with lambda similar to the premix engine or the diesel engine depending upon how the gaseous fuel is introduced to the combustion chamber, for example whether it is introduced upstream of an intake valve or directly introduced into the combustion chamber.
Known techniques for air-fuel ratio control comprise adjusting fueling quantity along with manifold absolute pressure (MAP) and air charge temperature (ACT). For example, U.S. Pat. No. 6,273,076, issued Aug. 14, 2001 to Beck et al., discloses a method of optimizing excess air ratio in a diesel-cycle engine on a full time, full range basis. In diesel-cycle engines, which operate lean, the demanded torque is achieved by controlling the fueling quantity. During a transient condition when the operator is demanding a new torque the fueling quantity changes, and in the steady state condition when there is no change in demanded torque the excess air ratio is controlled by adjusting the air supply. The method disclosed by Beck includes a step of adjusting a fuel quantity when the difference between an optimum lambda value and an actual lambda value (the error signal) is large. This indicates transient engine operation, for example a sudden and sharp increase or decrease in commanded power. After adjusting the fuel quantity, or instead of adjusting fuel quantity when there is no transient engine condition, the method includes the step of adjusting the air supply to minimize the error signal.
U.S. Pat. No. 7,270,089, issued Sep. 18, 2007 to Hoi Ching Wong, discloses a method of controlling excess air ratio when transitioning between operating modes of a dual fuel engine. The engine is preferably configured to operate in the pilot ignited gaseous fuel mode during as much of its speed and load range as practical except under (1) low speed and high load conditions and (2) light load and all speed conditions where it operates in diesel mode. During transition the method of Wong takes gas lambda in pilot ignited gaseous fuel mode and diesel lambda in diesel mode into consideration during transition between operating modes to avoid undesired or premature switching between modes. Other techniques for optimizing and controlling Lambda include skip fire techniques where for lighter loads some cylinders can be deactivated such that more fuel is injected in the remaining active cylinders which allows the lambda value to be decreased in these cylinders.
Conventional techniques of excess air ratio control do not teach precise control of lambda in a throttled engine operating near stoichiometry and where a premixed charge of natural gas is introduced into the cylinder and ignited by a pilot fuel. In such an engine there is a need for an excess air ratio control technique that allows lambda to be controlled when disproportionate amounts of fuel are consumed. For example, a preferred control technique exercises control over lambda when under light load conditions the fuel composition comprises 10% natural gas and 90% pilot fuel and under heavy load conditions when the fuel composition comprises 95% natural gas and 5% pilot fuel. Over the range of engine operating conditions the preferred control technique provides fast transient response and stable steady state operation to maintain lambda at or near the desired value.
The state of the art is lacking in techniques for closed loop fuel control in an engine that consumes more than one fuel type simultaneously and where the variability in the quantity of one fuel is much greater than the variability in the quantity of the other fuel. The present method and apparatus provide an improved technique for controlling air-fuel ratio in a multi-fuel internal combustion engine.